The field of clinical magnetic resonance imaging (MRI) depends for its success on the generation of strong and pure magnetic fields. A major specification of the static field in MRI is that it has to be substantially homogeneous over a predetermined region, known in the art as the “diameter spherical imaging volume” or “dsv.” Errors less than 20 parts per million peak-to-peak (or 10 parts per million rms) are typically required for the dsv. The uniformity of the field in the dsv is often analyzed by a spherical harmonic expansion.
The basic components of a typical magnetic resonance system for producing diagnostic images for human studies include a main magnet (i.e., a superconducting or non-superconducting magnet which produces the substantially homogeneous magnetic field (the B0 field) in the dsv), one or more shim magnets, a set of gradient coils, and one or more RF coils. Discussions of MRI, including magnet systems for use in conducting MRI studies, can be found in, for example, Mansfield et al., NMR in Imaging and Biomedicine, Academic Press, Orlando, Fla., 1982, and Haacke et al., Magnetic Resonance Imaging: Physical Principles and Sequence Design, John Wiley & Sons, Inc., New York, 1999. See also Crozier et al., U.S. Pat. No. 5,818,319, Crozier et al., U.S. Pat. No. 6,140,900, Crozier et al., U.S. Pat. No. 6,700,468, Dorri et al., U.S. Pat. No. 5,396,207, Dorri et al., U.S. Pat. No. 5,416,415, Knuttel et al., U.S. Pat. No. 5,646,532, and Laskaris et al., U.S. Pat. No. 5,801,609, the contents of which are incorporated herein by reference in their entireties.
In modern medical imaging, there is a distinct and long-felt need for smaller magnetic resonance systems. The typical aperture of a conventional MRI machine is a cylindrical space having a diameter of about 0.6-0.8 meters, i.e., just large enough to accept the subject's shoulders, and a length of about 2.0 meters or more. The dsv for such systems is located near the center of the aperture, which means that it is typically about a meter from the end of the aperture.
Not surprisingly, many people suffer from claustrophobia when placed in such a space. Also, the one-meter distance between the portion of the subject's body which is being imaged and the end of the magnet system means that physicians cannot easily assist or personally monitor a subject during an MRI procedure.
In addition to its effects on the subject, the size of the magnet is a primary factor in determining the cost of an MRI machine, as well as the costs involved in the siting of such a machine. In order to be safely used, MRI machines often need to be shielded so that the magnetic fields surrounding the machine at the location of the operator are below FDA-specified exposure levels. By means of shielding, the operator can be safely sited much closer to the magnet than in an unshielded system. Larger magnets require more shielding and larger shielded rooms for such safe usage, thus leading to higher costs.
Extremity MRI (also known as orthopedic MRI) is one of the growth areas of the MRI industry, with 20% of all MRI procedures in the United States in 2002 being performed on upper (e.g., arms, wrists, and elbows) and lower (e.g., legs, ankles, and knees) extremities. Extremity MRI systems are much smaller than whole-body or conventional MRI systems and are much easier to site, due both to their reduced size and reduced stray fields. They are therefore a low cost solution to the imaging of extremities.
While extremity MRI systems have a number of advantages to the subject and the operator, they represent a challenge in terms of the space available for the various coils making up the magnet and in terms of cooling those coils, whether they be superconducting or resistive coils. The close spacing between coils can also lead to high peak fields in some circumstances, as well as to substantial inter-coil and intra-coil stresses.
The present invention is directed to providing magnets which address these and other challenges of extremity MRI systems.